Expressions of quantum efficiency of resonant-cavity-enhanced (RCE) PIN photodetectors reported in the literature are based on the assumption of constant reflectivities of the quarter-wave stacks (QWS) at the ends of the cavity. The quantum efficiency is formulated in a closed analytical form that includes the structural parameters of the photodetector and takes into account the wavelength dependence of the reflectivities and the active region absorption coefficient. The variation of the QWS reflectivity and, in particular, its phase constant with wavelength has a significant influence on the resulting quantum efficiency spectra, as demonstrated in this paper. The results are in very good agreement with recently published experimental data which show a dominant peak at the operating wavelength. This behavior has not been predicted by previous simulation results. Since the quantum efficiency spectra are not periodic, the use of the finesse, defined as the ratio of the free spectral range to the full width at half maximum, as a measure of wavelength selectivity is not valid. The conventional quality factor definition used for filter design is thus adopted as a measure of selectivity. A genetic algorithm-based optimization and design procedure for RCE photodetectors have also been developed with the quantum efficiency, quality factor, and frequency bandwidth as input design parameters.